The application is based on two basic principles: modular audio chains that let you connect various instruments and virtual segments to make music with, and a timeline where you can set input patterns to annotate different melodies.
Vermilion Desktop Composer is an interesting application. If you are patient and give it time, you will see it can achieve very good results (you can check it out in the demo song).
Unfortunately, it's not very intuitive. You'll need a lot of practice before you actually reach that level.